Integrated circuits and methods of design and manufacture thereof

ABSTRACT

Integrated circuits and methods of manufacture and design thereof are disclosed. For example, a method of manufacturing includes using a first mask to pattern a gate material forming a plurality of first and second features. The first features form gate electrodes of the semiconductor devices, whereas the second features are dummy electrodes. Based on the location of these dummy electrodes, selected dummy electrodes are removed using a second mask. The use of the method provides greater flexibility in tailoring individual devices for different objectives.

TECHNICAL FIELD

The present invention relates generally to the fabrication of integratedcircuits, and more particularly to fabrication of semiconductor devicesusing lithography techniques.

BACKGROUND

Generally, semiconductor devices are used in a variety of electronicapplications, such as computers, cellular phones, personal computingdevices, and many other applications. Home, industrial, and automotivedevices that in the past comprised only mechanical components now haveelectronic parts that require semiconductor devices, for example.

Semiconductor devices are manufactured by depositing many differenttypes of material layers over a semiconductor workpiece or wafer, andpatterning the various material layers using lithography. The materiallayers typically comprise thin films of conductive, semi-conductive andinsulating materials that are patterned and etched to form integratedcircuits (ICs). There may be a plurality of transistors, memory devices,switches, conductive lines, diodes, capacitors, logic circuits, andother electronic components formed on a single die or chip, for example.

There is a trend in the semiconductor industry towards reducing the sizeof features, e.g., the circuits, elements, conductive lines, and vias ofsemiconductor devices, in order to increase performance of thesemiconductor devices, for example. The minimum feature size ofsemiconductor devices has steadily decreased over time. However, asfeatures of semiconductor devices become smaller, it becomes moredifficult to pattern the various material layers because of diffractionand other effects that occur during a lithography process. For example,key metrics such as resolution and depth of focus of the imaging systemsmay suffer when patterning features at small dimensions.

Innovative process solutions have been developed that overcome some ofthese limitations. However, many such process solutions also interactwith subsequent steps and may degrade other equally important factors.

For example, another goal of the semiconductor industry is to continueincreasing the speed of individual devices. Enhancing mobility ofcarriers in the semiconductor device is one way of improving devicespeed. One technique to improve carrier mobility is to strain (i.e.,distort) the semiconductor crystal lattice near the charge-carrierchannel region. Transistors built on strained silicon, for example, havegreater charge-carrier mobility than those fabricated using conventionalsubstrates.

One technique to strain silicon is to introduce stressor materials.Stressor materials exert strain on the channel of a device by variousmeans. Examples of such methods include lattice mismatch, thermalexpansion mismatch during thermal anneal, and/or intrinsic film stress.A typical transistor fabricated today comprises all these elements. Theuse of SiGe source/drain regions is an example of using lattice mismatchfor producing strain. Examples of thermal mismatch and film stressinclude stress memorization layers and contact etch stop layers.

One challenge with strain techniques arises from their layout effects.Channel strain not only depends on the stressor material, but also onthe location and placement of these materials. Hence, any modificationsmade for example, in the printing of these features during thelithography steps can seriously impact transistor performance and henceproduct performance.

Solving such interactions requires cross-functional development withinformation and knowledge sharing between different organizations. Whatare needed in the art are methods of leveraging lithography to enhancedesign and manufacturing processes.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by preferred embodiments ofthe present invention which provide integrated circuits, and methods ofdesign and manufacture thereof.

In accordance with an embodiment of the present invention, a method ofmanufacturing an integrated circuit includes depositing a gate materialon a semiconductor substrate and using a first mask to pattern the gatematerial, thereby forming a plurality of first and second features. Thefirst features form gate electrodes of the semiconductor devices,whereas the second features comprise dummy electrodes. Selected secondfeatures are removed using a second mask based on their location.

The foregoing has outlined rather broadly features of embodiments of thepresent invention in order that the detailed description of theinvention that follows may be better understood. Additional features andadvantages of embodiments of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiments disclosed may be readily utilized as a basisfor modifying or designing other structures or processes for carryingout the same purposes of the present invention. It should also berealized by those skilled in the art that such equivalent constructionsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIGS. 1 a and 1 b illustrate an embodiment of an integrated circuit,wherein FIG. 1 a illustrates a cross-sectional view and FIG. 1 billustrates a top view;

FIGS. 2 a and 2 b illustrate top views of lithography mask layers inaccordance with an embodiment of the present invention, wherein FIG. 2 aillustrates a first mask layer and FIG. 2 b illustrates a second mask orerase mask layer;

FIGS. 3 a-3 h illustrate cross-sectional views of a region of anintegrated circuit during various process steps of manufacturing usingembodiments of the invention;

FIGS. 4 a-4 c illustrate top views of a region of an integrated circuitduring various process steps of manufacturing using embodiments of theinvention;

FIG. 5 illustrates a flow chart of the process steps of manufacturing anintegrated circuit in accordance with an embodiment of the invention;

FIG. 6 illustrates a flow chart describing the flow of information ingenerating a layout in accordance with an embodiment of the invention;

FIG. 7 illustrates a flow chart of an embodiment of the invention ingenerating a layout;

FIG. 8 illustrates a flow chart of an embodiment of the invention ingenerating a layout;

FIG. 9 illustrates a flow chart of embodiments of the invention ingenerating a layout for implementing the selective double patterningtechnique, wherein FIG. 9 a illustrates a layer based description, FIG.9 b illustrates a rules-based description, and FIG. 9 c illustrates amodels-based description;

FIG. 10 illustrates a flow chart of an embodiment of the invention ingenerating a layout;

FIGS. 11 a-11 b illustrates an embodiment of an integrated circuitmanufactured using embodiments of the invention, wherein FIG. 11 aillustrates the cross-sectional view and FIG. 11 b illustrates the topview;

FIGS. 12 a-12 b illustrates an embodiment of an integrated circuitmanufactured in accordance with embodiments of the invention, whereinFIG. 12 a illustrates the cross-sectional view and FIG. 12 b illustratesthe top view;

FIGS. 13 a and 13 b illustrates a cross-sectional view of an embodimentof an integrated circuit manufactured in accordance with embodiments ofthe invention.

FIG. 14 shows a cross-sectional view of an embodiment of an integratedcircuit manufactured in accordance with embodiments of the invention;

FIGS. 15 a-15 b illustrates cross-sectional views of embodiments of anintegrated circuit manufactured in accordance with embodiments of theinvention, wherein each embodiment illustrates a local variation in theplacement of dummy features, wherein FIG. 15 a illustrates an isolateddevice and FIG. 15 b illustrates a dense device; and

FIGS. 16 a-16 b show cross-sectional views of a region of the integratedcircuit during various process steps of manufacturing in embodiments ofthe invention, wherein each embodiment illustrates an alternate use of aselective double patterning technique in forming asymmetric devices.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the preferredembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

A phenomenon referred to as proximity effect poses a primary challengein transferring patterns during lithography. Proximity effects result invariation of line width of patterns, depending on the proximity of afeature to other features. Proximity effects arise during, for example,imaging, resist patterning, or subsequent transferring of the pattern,such as during etching. To first order, the magnitude of the effecttypically depends on the proximity or closeness of the two featurespresent on the mask. However, proximity effects can extend to longerdistances extending to several micrometers especially for etchprocesses.

One of the reasons for the observed proximity effects arises fromoptical diffraction. Hence, adjacent features interact with one anotherto produce pattern-dependent variations. For example, for lithographicexposure, closely-spaced dark features (densely packed gates) tend to bewider into a positive tone resist than widely-spaced features (forexample isolated gates), although on a lithography mask they comprisethe same dimension. Similarly, during etch processes, the reverse istrue, and hence closely-spaced features tend to be transferred smallerthan widely-spaced features. It is important in many semiconductordevice designs for features to have uniform, predictable dimensionsacross a surface of a wafer, for example, to achieve the required deviceperformance.

To compensate for such proximity effects, optical proximity corrections(OPC) are applied to mask layouts of lithographic photomasks, which mayinvolve adjusting the widths or lengths of the lines on the mask.Advanced methods of OPC correct corner rounding and a general loss offidelity in the shape of features by adding small secondary patternsreferred to as serifs to the patterns.

Finally, sub resolution assist features, also called scatter bars, arealso added, which are features formed on the mask but are not patternedor printed. For example, sub resolution assist features typicallycomprise a plurality of lines significantly thinner than the minimumpatternable width or resolution of the exposure tool. These assistfeatures effectively change the pattern density and help improve depthof focus of the exposure system. Consequently, these assist featuresimprove uniformity in printing features of different density forexample, between isolated and dense lines.

The use of scatter bars, however, is becoming increasingly difficult toimplement. For example, the width of the scatter bars must besignificantly smaller than the critical dimension of the minimum featureto avoid printing. Shrinking the critical dimension also requiresshrinking the widths of scatter bars. Thus, increasing the difficulty ofincorporating these features into the mask as well as their subsequentinspection and repairs.

Further, the patterning of ever shrinking minimum features andespecially pitches requires aggressive increases in numerical apertureof the lithography system. Although higher numerical aperture increasesresolution, the depth of focus degrades considerably. Consequently, theinclusion of sub resolution assist features is not sufficient to improvedepth of focus to a reasonable range suitable for production of futuresemiconductor nodes. However, further improvements in depth of focus canbe made if the sub resolution assist features are allowed to print. Suchfeatures, also called printing assist features, are now being explored.

Printing assist features, also called dummy gates, are typicallyintroduced in the layout to improve the quality of the transfer ofneighboring, electrically active gates. These dummy gates would betransferred to the final chip layout on the wafer just like theneighboring, electrically active gates. For example, additional gatelines may be printed, for example, over isolation regions. Such dummygate lines reduce the difference in pitch between wider and narrowerpitch structures. However, the use of such dummy features introducesdrawbacks due to the presence of these structures in the final layout orproduced chip. For example, the presence of these additional dummystructures may in some cases reduce the electrical performance of thechip.

Another method of advantageously using printing assisted features isprovided by a technique called double patterning. In double patterning,the mask contains a number of additional features. These additionalfeatures such as dummy gate lines are printed along with criticalfeatures. The additional features, however are removed in a subsequentprocess step by exposing these additional features to a second maskstep.

The use of printing assist features enables optimization of lithographyprocess conditions to increase the common process window. For example,densely packed gates can be patterned in regions assigned to form bothisolated transistors and densely packed transistor arrays. Hence, theisolated transistor region comprises the active gate line and aplurality of printing assist features or dummy gate lines. The isolatedtransistors are subsequently formed by the removal of these dummy gatelines.

In the typical double patterning process, all the additional featuresare removed during the second process step. However, in some cases, itmight be better to selectively remove some of these additional features.For example, channel strain is used to boost performance. As discussedpreviously, channel strain in turn may depend on gate to gate spacing.Hence, the presence or absence of dummy gate lines may modulate strainin the channel. In most cases, the increase in gate to gate spacingincreases channel strain and hence transistor performance. If the solecriterion is to increase channel strain, it would be beneficial toremove all the dummy gate lines. However, many other factors are alsoimportant. For example, in structures or circuits containing transistorarrays only the corner or edge transistors are affected. In fact, suchedge transistor difference may result in deleterious variation and maynot be preferred in some embodiments. Further, a strong strain gradientis created within the silicon moat region. The combination of higherstrain along the moat corners along with the high strain gradients mayincrease propensity to form defects such as dislocations.

Similarly, removal of printing assist features may create other problemssuch as erosion of process margins due to, for example, loading effects.For example, a number of process steps are pattern density dependent,i.e., they behave differently depending on the local density, forexample, of gate lines. Examples of such processes include etchprocesses particularly wet etch, planarization processes such aschemical mechanical polishing, and deposition processes such as copperelectrochemical deposition. Such variation may dramatically increasewithin wafer variation and in some instances may result in productfailure.

Hence, it may be advantageous to selectively remove only a portion ofthe printing assist features referred to hereafter as a selective doublepatterning technique. Addition or deletion of printing assist featureshas to be carefully optimized in view of various factors. One embodimentof this invention is to provide an algorithm to remove printing assistfeatures to maximize impact of strain on circuit or product performance,while minimizing deleterious effects such as process yield.

In various embodiments, the present invention teaches the localmodulation in electrical behavior using a selective double patterningtechnique. These modulations typically are performed along withoptimization of other parameters such as layout area, process margin,parametric yield, or process window.

Embodiments of the present invention achieve technical advantages byproviding a method to simultaneously increase process yield and productperformance using a selective double patterning technique. The presentinvention will be described with respect to preferred embodiments in aspecific context, namely selectively removing printing assist featuresin field effect devices. The invention may also be applied, however, toother types of devices such as diodes, bipolar junction transistors,thyristors, memory devices such as DRAM, FeRAM, phase change memories,or floating gate devices. Similarly, the invention may also be appliedto other types of devices in other applications and other technologicalfields. Embodiments of the invention may be implemented in many types ofsemiconductor devices, such as logic, memory, peripheral circuitry,power applications, and other types of semiconductor devices, asexamples.

An exemplary integrated circuit manufactured using embodiments of thecurrent invention is shown in FIG. 1 and various alternate embodimentsof the integrated circuit are shown in FIGS. 9-14. An exemplary mask forforming the semiconductor device is shown in FIG. 2. An exemplary methodusing embodiments of the current invention will then be described withrespect to the cross-sectional views of FIGS. 3 a-3 h and FIGS. 4 a-4 c,and the flow charts of FIG. 5. FIGS. 6-8 describe embodiments of thecurrent invention describing generation of layout designs of theintegrated circuit.

FIGS. 1 a and 1 b illustrate cross-sectional and top-down views of asemiconductor device manufactured in accordance with a preferredembodiment of the present invention. With reference now to FIG. 1 a, thetransistor regions 100 and 200 comprise transistor arrays 100 acontaining transistors 101-103 and 201 disposed between isolationregions 26 in a substrate 99. The silicon area of region 200 containssufficient area for a single transistor also called an isolatedtransistor. The silicon area of region 100 typically contains aplurality of transistors of a given critical dimension L. The distance pbetween the transistors in the region 100 determines the pitch. If thisdistance p approaches the smallest allowed distance for the giventechnology, the transistors 101-103 are also called minimum pitchtransistors. A typical integrated circuit contains both minimum pitchtransistors, such as transistors 101-103, and isolated transistors, suchas transistor 201.

The transistors 101-103 and 201 comprise channel regions 18 disposed inactive regions 1, gate electrodes 10, an isolating material formingisolation regions 26, source/drain regions 55/57, and spacers 38. Astrain inducing etch stop layer 41 is present over the source 55, drain57, gate oxide 71 and gate electrode 10, although this may not be astrain layer in some embodiments. Further, the gate lines 10 are formedadjacent to additional dummy gate regions 20. The dummy gate regions 20are formed in the first transistor regions 100 but not the secondtransistor regions 200. The transistors in the two regions 100 and 200are formed identically except for the presence of the dummy gate regions20. As will be described using various embodiments, the dummy gates onregion 200 have been selectively removed by the selective doublepatterning technique.

The effective gate pitch defined as the distance between adjacent gatelines (10 to 10) or (10 to 20) defines the strain in the channel regionof the transistor. Typically, channel strain decreases as effectivepitch is reduced. As the transistors 101 and 201 have differenteffective gate pitches, they operate at different “ON” currents orelectrical performance. The transistor region 200 comprises a large gatepitch and hence is optimized for performance, whereas the transistorregion 100 comprises a tight gate pitch and is optimized for processvariation arising, for example, from gate density variation.

FIGS. 2 a and 2 b provide illustrations of a mask used in the selectivedouble patterning process to manufacture, for example, the transistorregions 100 and 200. As discussed earlier, a typical double patterningprocess comprises a first expose, develop and etch followed by a secondexpose, develop and etch, forming the final pattern. In particular, FIG.2 a shows the mask layer 499 used in the formation of the firstexposure. FIGS. 2 a and 2 b also show the areas forming the transistorregions 100 and 200. The first exposure comprises openings 500 in themask exposing the photoresist. The regions 501 are opaque to radiationand hence are not developed. The first exposure is used to pattern gatelines as well as form printing assist features. The illuminationconditions are selected to maximize the image quality, for example,minimize across chip line width variation, over a range of processwindow parameters such as depth of focus, exposure dose and mask errorfactor.

A second mask, as shown by the second mask level 509 over the transistorregions 100 and 200 exposes regions 510 selectively. The second mask isan erase or cut mask in that it removes selected regions 510. Forexample, the second mask level 509 contains transparent mask regions 510and opaque regions 511 that block exposure of any underlying resist. Thecombination of the first and second exposure results in selectivelyplaced gate lines as shown for example in FIG. 1 or FIGS. 9-13.

By minimizing variation in spacings and openings of printing during thefirst mask step, variations due to pattern density are minimized. Assecond mask level 509 is an erase mask, it does not need the stringentrequirements as for the first mask level 499.

The mask design has been explained in terms of opaque and transparentregions to clearly describe the embodiments of the invention. However,actual mask design and materials can be chosen to incorporatemodifications to improve the imaging system. For example, to improveimage resolution, the mask design may comprise attenuated phase shiftermaterials in regions 501 and transparent materials in region 500 of FIG.2 a. Similarly, to improve depth of focus, the mask design for eachlayer may comprise OPC features such as hammerheads, serifs, subresolution assist features, etc. Further, although a positive resist hasbeen assumed, the transistor regions 100 and 200 can be fabricated usinga negative resist (the mask design could be suitably adjusted).

FIGS. 3 a-3 g provide cross-sectional diagrams illustrating a firstembodiment method of the present invention using the mask layers 499 and509 of FIG. 2. FIGS. 4 a-4 c illustrate an associated top view of theimplementation, and FIG. 5 illustrates an associated flow diagram of oneimplementation of the process. For clarity, the process flows andfeatures of FIG. 3 illustrate only one type of transistor region (e.g.,regions 200 of FIG. 1). In FIG. 4, the transistor regions 100 and 200(e.g., of FIG. 1) will be illustrated at selected process steps. Whilecertain details may be explained with respect to only one of theembodiments, it is understood that these details can also apply to otherones of the embodiments.

Referring first to FIG. 3 a, a semiconductor body 99 is provided. In thepreferred embodiment, the semiconductor body 99 is a silicon wafer. Someexamples of the body 99 are a bulk mono-crystalline silicon substrate(or a layer grown thereon or otherwise formed therein), a layer of (110)silicon on a (100) silicon wafer, a layer of a silicon-on-insulator(SOI) wafer, or a layer of a germanium-on-insulator (GeOI) wafer. Inother embodiments, other semiconductors such as silicon germanium,germanium, gallium arsenide, indium arsenide, indium gallium arsenide,indium antimonide or others can be used with the wafer.

In the first embodiment, isolation trenches 28 are formed in thesemiconductor body 99 as shown in FIG. 3 a. The isolation trenches 28can be formed using conventional techniques. The trenches 28 defineactive area 1, in which integrated circuit components can be formed. Thetrenches 28 are then filled with an isolating material forming isolationregions 26. The trench fill can be a single material or multiplematerials. Examples of trench fill materials include oxides such asthermal oxide, High Density Plasma (HDP) oxide, HARP oxide, TEOS oxidesand various nitrides. In other embodiments, other trench fillingprocesses can be used. For example, while the trench is typically lined,this step can be avoided with newer fill materials (for example HARP™).The top surface of the semiconductor body 99 is subsequently polishedand planarized. Chemical mechanical polishing (CMP) is a specificexample of the polishing process.

This completes the formation of the isolation regions 26. The top viewcross section of transistor regions 100 and 200 are shown at this stageof the fabrication process in FIG. 4 a.

FIG. 3 a shows the device after gate stack deposition. After STIformation, well, punch through and threshold implants are performed. Agate dielectric 71 is deposited over exposed portions of thesemiconductor body 99. In one embodiment, the gate dielectric 71comprises an oxide (e.g., SiO₂), a nitride (e.g., Si₃N₄), or acombination of oxide and nitride (e.g., SiON, or an oxide-nitride-oxidesequence). In other embodiments, a high-k dielectric material having adielectric constant of about 5.0 or greater is used as the gatedielectric 71. Suitable high-k materials include HfO₂, HfSiO_(x), Al₂O₃,ZrO₂, ZrSiO_(x), Ta₂O₅, La₂O₃, nitrides thereof, HfAlO_(x),HfAlO_(x)N_(1−x−y), ZrAlO_(x), ZrAlO_(x)N_(y), SiAlO_(x),SiAlO_(x)N_(1−x−y), HfSiAlO_(x), HfSiAlO_(x)N_(y), ZrSiAlO_(x),ZrSiAlO_(x)N_(y), combinations thereof, or combinations thereof withSiO₂, as examples. Alternatively, the gate dielectric 71 can compriseother high-k insulating materials or other dielectric materials. Asimplied above, the gate dielectric 71 may comprise a single layer ofmaterial, or alternatively, the gate dielectric 71 may comprise two ormore layers.

The gate dielectric 71 may be deposited by chemical vapor deposition(CVD), atomic layer deposition (ALD), metal organic chemical vapordeposition (MOCVD), physical vapor deposition (PVD), or jet vapordeposition (JVD), as examples. In other embodiments, the gate dielectric71 may be deposited using other suitable deposition techniques. The gatedielectric 71 preferably comprises a thickness of about 10 Å to about 60Å in one embodiment, although alternatively, the gate dielectric 71 maycomprise other dimensions. In the illustrated embodiment, the samedielectric layer would be used to form the gate dielectric 71 for boththe p-channel and n-channel transistors. This feature is however notrequired. In alternate embodiments, p-channel transistors and n-channeltransistors could each have different gate dielectrics.

The gate electrode layer 10 is formed over the gate dielectric 71. Thegate electrode layer 10 preferably comprises a semiconductor material,such as polysilicon or amorphous silicon, although alternatively, othersemiconductor materials may be used for the gate electrode layer 10. Inother embodiments, the gate electrode layer 10 may comprise TiN, TiC,HfN, TaN, TaC, W, Al, Ru, RuTa, TaSiN, NiSi_(x), CoSi_(x), TiSi_(x), Ir,Y, Pt, Ti, PtTi, Pd, Re, Rh, borides, phosphides, or antimonides of Ti,Hf, Zr, TiAlN, Mo, MoN, ZrSiN, ZrN, HfN, HfSiN, WN, Ni, Pr, VN, TiW, apartially silicided gate material, a fully silicided gate material(FUSI), other metals, and/or combinations thereof, as examples. In oneembodiment, the gate electrode layer 10 comprises a doped polysiliconlayer underlying a silicide layer (e.g., titanium silicide, nickelsilicide, tantalum silicide, cobalt silicide, or platinum silicide).

The gate electrode layer 10 may comprise a plurality of stacked gatematerials, such as a metal underlayer with a polysilicon cap layerdisposed over the metal underlayer. A gate electrode layer 10 having athickness of between about 400 Å to 2000 Å may be deposited using CVD,PVD, ALD, or other deposition techniques.

A resist layer 311 is deposited over the gate electrode layer 10. Theresist layer 311 may be either organic or inorganic. Some examples ofinorganic resist layer 311 include silicon dioxide, silicon nitride,silicon oxy-nitride, titanium nitride and/or a SILK (silicon-containinglow-k) layer. The resist layer 311 may also be an organic layer such asa bottom anti-reflective coating (BARC) layer (such as polymides, andpolysulfones), a FLARE layer, and/or a BCB layer. The resist layer 311may optionally be baked to form a hard baked, thermally or chemicallycross-linked resist. Finally, although only a single layer of resist 311is shown, the resist layer 311 may comprise multiple layers. Forexample, in some embodiments, the resist layer 311 may be a bilayer ortrilayer film comprising different materials.

A photo-resist 315 is deposited on the gate stack. The photo-resistlayer 315 is a resist that can be developed by exposure to radiationsuch as deep UV radiation used by lithography systems. In preferredembodiments, this photo-resist 315 is sensitive to 193 nm or 157 nmelectromagnetic radiation. The resist used may either be positive ornegative. Examples of resist polymers are poly-p-hydroxystyrene,acrylates, novolak or cycloaliphatic copolymers.

Referring to FIG. 3 b, the photo-resist 315 is exposed using the firstmask level 499 of FIG. 2 a. The first mask level 499 of FIG. 2 acomprises the desired gate features, but also includes the additionalprinting assist features. Thus, regions 420 are exposed whereas regions421 remain unexposed.

Referring to FIG. 3 c, the exposed photo-resist regions 420 are removedby etching. Using the photo-resist regions 421 as patterns, ananisotropic etch such as a reactive ion etch is used to remove theexposed portion of the resist 311. The top views of transistor regions100 and 200 (e.g., of FIG. 1 a) are shown at this stage of thefabrication process in FIG. 4 b.

A second photo-resist layer 316 is coated over the semiconductor body 99covering the exposed gate electrode layer 10 and the photo-resist 315 asshown in FIG. 3 d. Referring next to FIG. 3 e, the photo-resist 316 isexposed using the second mask level 509 of FIG. 2 b forming exposedregions 430 and unexposed regions 431. Although in the currentembodiment, the second mask or erase mask is exposed in a process stepfollowing the first mask exposure, in alternate embodiments, the secondmask step may be performed during subsequent processing. Further,although the second mask step is performed after the first mask step inthe current embodiment, the second mask may also be performed before thefirst mask step in some embodiments.

The exposed regions 430 are developed and etched out as shown in FIG. 3f. Further, the resist 311 and photo resist 315 (if remaining) are alsoetched and removed. As shown in FIG. 3 g, the pattern is transferred togate electrode layer 10 and gate oxide 71 by reactive ion etch. Anadditional trim etch may be performed at this stage to reduce thecritical dimension of the gate lines 10 further, before removal of thetop resist layer. Finally, as shown in FIG. 3 h, all the resist layersare removed forming the patterned gate electrode line 10.

The top cross-section of the device fabricated at this stage of theprocessing is shown in FIG. 4 c. The device now comprises dummy gatelines 20 and gate lines 10 in transistor region 100. The transistorregion 200 comprises only gate lines 10.

After gate stack formation, further processing continues as perconventional flow. For example, spacers, extensions, source/drainregions, silicide regions, contacts and metallization including vias andmetal lines may be formed completing the fabrication of the integratedcircuit.

An etch step was used in the current embodiment to transfer the imagebefore the second exposure step. In alternate embodiments, the twoexposure steps may be performed concurrently without an intermediateetch step. In such embodiments, the final image is formed on thephoto-resist by the combination of the first exposure and secondexposure processes.

So far, various embodiments have illustrated the semiconductor deviceand methods for forming the device. Various embodiments exist in theimplementation of the selective double patterning method in generating alayout and a mask. In various embodiments illustrated in FIGS. 6-10, afinal layout is generated with designs optimally designed for each masklayer of the selective double patterning process.

An embodiment of the design methodology using the selective doublepatterning in layout design will now be described using the flow chartof FIG. 6. Design 825 and manufacturing 930 comprise two primary aspectsof development of an integrated circuit. In its simplest form, a chip isdesigned by the designer and a layout comprising the functionality istaped out to the manufacturing engineers. The manufacturing engineers inturn take the taped out layout and produce a final chip performing theoperation envisioned. The presence of selective double patterningprocess 809 presents a unique opportunity for designers andmanufacturing engineers. For example, at the device level, engineers canuse the information on next neighboring dummy line or spacing tooptimize a given transistor performance. Similarly, at a circuit level,the layout may be optimized to also include proximity effects arisingfrom interactions of neighboring dummy regions. Such optimizations arepossible only by careful flow of information about the selective doublepatterning technique to engineers doing various operations.

In a modern integrated chip development, design and manufacturingdivision have several levels of information transferring between them.For example, designers at a block level will perhaps only use high levelinformation regarding the electrical functional of components from themanufacturing group. However, designers involved in physical layout willclearly require much more information. Hence, based on utility, thisinformation flow can be divided into several groups. Only as an example,the information from the selective double patterning technique 809 canbe passed onto designers at various levels. For example, the technologyspecification 810 which could be a design rule document or a descriptioncould comprise information regarding the selective double patterningtechnique 809. For example, this technology specification 810 couldinclude an allowable range of pitches, spacings, and critical dimensionsfor the first mask level. Similarly, the technology specification 810could also include an allowable range of overlap between the first andsecond mask layers to minimize misalignment errors, as well an allowablerange of openings. For example, this could be a function of the pitches,openings and spacings allowed for the first mask layer.

Further, device engineers, as well as compact model developmentengineers, would require information regarding the selective doublepatterning technique 809. Device engineers could use informationregarding the selective double patterning technique 809 to maximizedevice performance during device development 811. For example, afterdevice development 811, transistor performance from proximity to a dummygate line or density of dummy gate lines may be characterized into acompact circuit element model 985. As one example, the circuit elementmodel 985 could determine the channel mobility and hence the currentvoltage characteristics of transistors based on the local strain. Thelocal strain could in turn depend on the density of dummy or contactedgate lines surrounding the given transistor.

Similarly, the available options from the selective double patterningtechnique may be passed onto process models 991 that extract, forexample, product yield models. Such process models 991 could, forexample, use the density and dimensions of dummy and active gates in aparticular region to calculate potential hot-spots or yield lossregions.

Different design groups can be given the appropriate level ofinformation. Thus, the developed layout at chip tapeout 910 includes afully optimized product. The final layout can be optimized either for asingle component, for example performance, or a combination of factors,for example performance, process yield, process window, etc. Thedesigned layout may be decomposed into different layers such that eachlayer comprises different designs for each mask step (or exposure). Eachof these designs may be properly optimized for all requiredfunctionality. Although not discussed, each layer of the designundergoes optical proximity correction, subsequently the layout is sentfor mask generation 920.

An embodiment of design methodology using the selective doublepatterning technique 809 will be described using the flow chart of FIG.7. In designing integrated circuit chips, information is refinedprogressively to include more detail until a physical layout is made.

A functional description 800 of a product is progressively transformedinto, for example, a schematic 805. The schematic 805 comprises multiplelevels, wherein the lowest level is comprised of primitives such assingle devices and each upper layer comprises successively complexblocks. For example, the top component of the schematic is a blockgenerally describing the entire circuit at a gross level (i.e.controller, processor, etc.). The schematic 805 is comprised ofindividual devices such as resistors, transistors, capacitors, switches,etc. and other hierarchical blocks. A database or netlist 811 is thencreated of the schematic in which every device is listed, along with itsproperties, connectivity and proper dimensions to verify information(e.g., which device is connected to which other device). The netlist 811is still an abstract representation of the circuit. For instance, thenetlist 811 may be generated with some knowledge of the manufacturingprocess, for example, from the technology specification 810. It mayhence include numerical values for specific resistances, capacitancesetc of interconnects, transistors and other devices, to simulate forexample a standard cell or product. For example, the netlist 811 may beoptimized to deliver a certain delay (performance), active and standbypower etc. The netlist 811 is then translated into a layout 900 usingphysical design 850. Physical design 850 converts the abstractrepresentation into a physical representation. Physical design 850 mayinclude many steps such as floor-planning, place and route, compaction,and clock tree synthesis. A layout 900 of the integrated circuit iscreated after physical design 850. A state-of-the-art layout 900includes a collection of many levels of geometrical description of theIC. After the fabrication of the layout, the chip is taped out. Thedummy features from the selective double patterning technique 809 areplaced after chip tapeout 910 and appropriate masks are generated. Themasks are subsequently sent for manufacturing.

The transformation applied at each level in the design flow is generallyverified. The layout 900 is rigorously tested and verified to satisfyall metrics. Such verification can include functional correctness andtiming, among other performance metrics such as power consumption. Anextraction tool 950 reads the designed layout 900 to extract circuitelements, their electrical connectivity, and their parasitics. Aprocedure called layout-versus-schematic (LVS) 975 takes thisinformation along with circuit element models 985 to determine thefunctionality of the layout 900. This layout functionality is comparedwith the functionality of the schematic 805 or netlist 811 to determinethe validity of the design. These processes may be iteratively performeduntil all conditions are satisfied.

Both the physical design 850 and the LVS 975 tools can utilizeinformation from the selective double patterning technique 809 eitherdirectly, or as a rule or a model in the technology specification 810, acircuit element model 985 or in any other suitable way. Consequently,the final layout 900 contains optimization based on the knowledge of theselective double patterning technique 809 to optimize a user selectedset of product parameters.

The layout thus includes dummy features, for example, that areintentionally added but will be removed at a subsequent step. Forexample, during physical design 850 or layout generation 900, all oronly some dummy features may be added or marked for inclusion in asubsequent step. This information may be transferred to subsequent stepssuch as automated printing assist feature generation and layoutdecomposition but also to all the electrical analysis steps such as LVSusing different methods in various embodiments. In various embodiments,this may be accomplished by using extra layers (marking layers or designlayers or marking edges), rules-based descriptions (for example,measuring dimensions in the layout and/or analyzing the relationship toother existing design levels such as active area or contact levels), ormodel-based methods (process-based or based on electrical performancethat is modeled locally), as well as combinations of all or some ofthese methods.

This final layout 900 is taped out to manufacturing (chip tapeout 910).The layout 900 is processed to add additional dummy features during maskgeneration 920. These additional dummy features may be automaticallygenerated (automated dummy gate generation 911) before maskdecomposition 912. Such additional dummy features may be in addition tothe dummy structures added during the layout design process. Hence, invarious embodiments, all or some of the dummy features may be added atthis stage. The layout decomposition 912 not only requires informationregarding dummy features added to the first mask, but also requiresinformation on their selective removal to appropriately design thesecond mask (erase or removal mask). Similar to the case of addinginformation to form the dummy features, information regarding theirselective removal is transferred to layout decomposition 912, utilizingvarious methods, for example, as discussed above (extra layers, rules,models, etc.). The generated mask comprising the first mask and thesecond mask (erase or removal mask) is used in the manufacturing 930 ofthe chip.

An embodiment of design methodology using the selective doublepatterning technique 809 will be described using the flow chart of FIG.8. This embodiment adds the optimization of process yield to the flowchart described by FIG. 7. The yield extractor 995 reads the layout 900and extracts a yield based on process models 991 and technologyspecifications 810. The yield extractor 995 can itself have variouslevels of sophistication based on need and/or availability of models.For example, the yield extractor 995 may relate local variations inpattern density from the layout 900 and empirically extract a productyield using process models 991. Alternately, the yield extractor 995 maysimulate various process steps or combinations of process steps todetermine process yield. An objective function that optimizesperformance and yield together may be created. Both the yield extractor995 and the LVS 975 are simultaneously performed to modify the layout900. On successful optimization, a new layout 900 is generated thatmeets the specified criteria of co-optimizing process yield and productperformance.

An embodiment of design methodology using the selective doublepatterning technique 809 will be described using the flow chart of FIG.9, which includes FIGS. 9 a-9 c. The embodiments detail the markingoptions for implementing the selective double patterning technique. Theselective double patterning technique may be introduced in a number ofways. For example, in one embodiment, the dummy features may be drawn.The dummy features may also be marked for removal during subsequentsteps. Alternately, in various embodiments, the dummy features may beautomatically generated in a separate data preparation step after chiptapeout 910. An embodiment using selective automatic dummy generation911 is now discussed using FIG. 9 a.

The layout after chip tapeout 910 undergoes automatic dummy gategeneration 911 (see FIG. 7). This process may comprise the addition ofextra layers 1010. Alternately, additional layers may be added duringlayout design. For example, the layout may comprise design layers 1021and marking layers 1011. Design layers 1021 include features from thedesigned layout whereas marking layers 1011 include additionalinformation or data to be used by the dummy feature generation as wellas by the mask decomposition processes.

Marking layers 1011 define locations for generation of dummy features aswell as locations in which dummy features are not generated. Markinglayers 1011 also define the originally drawn or generated dummy featureschosen to be removed (or alternately kept) during the second removalprocess step. As illustrated in FIG. 9 a, marking layers 1011 mayinclude means to mark whole shapes or areas by using cover shapes 1013.Similarly, marking layers 1011 may additionally include information formarking the edges of shapes 1014. In each case, the edges or shapes maybe marked for either dummy generation or dummy removal. In variousembodiments, information from marking layers 1011 regarding dummygeneration may be used (for example, during mask decomposition) informing the first mask whereas information regarding dummy removal maybe used in forming the erase mask. Marking layers 1011, hence, includeinformation for each shape or edge and define regions in which togenerate or to not generate a dummy feature. Similarly, marking layers1011 also contain shapes or areas, and edges that are marked for dummyremoval (and hence also information on dummy features that are notremoved).

Design layers 1021 include features from the designed layout and includedummy features such as dummy gates 1022 and functional or active gates1023. Design layers 1021 may also include additional informationregarding these features. In various embodiments, design layers 1021additionally include information on neighboring features to the activegates 1023, such as neighboring dummy gates 1022. For instance, anactive gate 1023 located in the center of a stacked gate structure maynot contain information on neighboring dummies, whereas another activegate 1023 on the edge of the stacked gate structure may includeinformation on neighboring dummies. For example, design layers 1021 mayinclude information on neighboring dummy gates 1022 that are removed orkept. Alternately or additionally, design layers 1021 identify dummygates and their future status (e.g., removed).

In other embodiments, as illustrated in FIG. 9 b, the layout may bemarked using a rule based descriptions 1110. For example, as illustratedin box 1112, relationship, dimensioning, or interaction with otherdesign layer or neighboring regions may be used in formulating therules. As shown in box 1111, in some cases, the rules may be formulatedfor dimensioning based on neighbors within the same design layers.Similarly, as illustrated in FIG. 9 c, model-based descriptions 1210 maybe used in some embodiments. For instance, such descriptions may includelithography process based performance 1211, electrical performance 1212or yield performance 1213, etc. The layout is decomposed using thesemarking options and information regarding the selective doublepatterning technique and masks comprising at least a generation mask forcreating the dummy features and at least an erase mask for removing thedummy features is generated for manufacturing.

An embodiment of design methodology using the selective doublepatterning technique 809 will be described using the flow chart of FIG.10. This embodiment combines many of the interactions of the selectivedouble patterning technique 809 during various stages in the design andmanufacturing of an integrated chip.

As illustrated in FIG. 10, the dummy gate placement/removal scheme 1809may be implemented in various stages either during or after layoutdesign. The dummy gate placement/removal scheme 1809 also includesinformation from process specification 1810 containing the details ofthe process. As discussed, the dummy gate placement/removal scheme 1809is used either before or after chip tapeout 910. Each step may utilizeonly certain specific aspects of the dummy gate placement/removal scheme1809. The circuit specification 1800 and schematic design 1805 are firstgenerated. The chip is taped out undergoing several steps includinglayout design 1850 (which in turn depends on the process specification1810 through, for example, a design manual 1811), layout assembly 1901and LVS 975. The dummy features may be added (dummy gate placement 1911)after chip tapeout 910 using, for example, an automated step. Variouspost processing steps may be performed and the layout is decomposed toform a mask file suitable for mask generation (layout decomposition912). The mask set comprising a generation mask and an erase mask isgenerated after layout decomposition 912. The generated masks are usedin wafer processing 1913 that eventually forms functional gates 1914 andfunctional circuitry 1915. In the embodiment described in FIG. 10, thedummy gate placement/removal scheme 1809 may place and remove dummygates in a number of different ways. For example, in variousembodiments, all or some of the dummy gates may be placed during layoutdesign 1850. In some embodiments, all or some of the dummy gates may beplaced during dummy gate placement 1911. In various embodiments, thedummy gates may be placed during both layout design 1850 and dummy gateplacement 1911. Similarly, the dummy gates may be fully or partiallyremoved during wafer processing 1913, which is defined and coded intothe mask layout of the second mask during layout decomposition 912.

The dummy gate placement/removal scheme 1809 may be implemented invarious embodiments as a generic description understood by a supportingsoftware framework or data preparation tools. In some embodiments, thedummy gate placement/removal scheme 1809 may comprise a layout designmarking layer. In other embodiments, the dummy gate placement/removalscheme 1809 may comprise individual gate layers to distinguish betweendifferent types of gates. For example, functional gates may be dividedinto different layers based on neighboring gates. For instance, gatesthat are bounded by dummy gate neighbors may be separated into adifferent layer. Similarly, in some embodiments, the dummy gateplacement/removal scheme 1809 may comprise a rule, model, orfunctionality based description. In some embodiments, the dummy gateplacement/removal scheme 1809 may be a suitable combination of any ofthe above schemes.

Embodiments of a semiconductor device manufactured in accordance withembodiments of the invention will now be described using FIGS. 11-16.

First, an embodiment of the semiconductor device manufactured inaccordance with an embodiment of the present invention is described inFIG. 11, and includes FIGS. 11 a and 11 b, wherein FIG. 11 a illustratesa cross sectional view and FIG. 11 b illustrates the top view. FIG. 11illustrates transistor array 100 a containing minimum pitch transistors101-105 in transistor region 100 and isolated transistors 201 intransistor region 200, wherein the transistor region 200 contains dummygate regions 210 and 211, whereas the transistor region 100 does not.The dummy regions 110 and 111 in region 100 (shown by dashed lines) havebeen selectively removed using the erase mask 509 of FIG. 2 b. Forexample, this may be done to minimize the performance difference betweenisolated and minimum pitch devices. The isolated transistors in thepresence of the dummy gates 20 behave similarly to minimum pitchdevices.

An embodiment of the semiconductor device manufactured in accordancewith an embodiment of the present invention is described in FIG. 12,which includes FIGS. 12 a and 12 b, wherein FIG. 12 a illustrates thecross sectional view and FIG. 12 b illustrates the top view. In FIG. 12,the transistor regions 100 and 200 comprise isolated transistors 101 and201 of length L and width W. The dummy regions 210 and 211 in transistorregion 200 (shown by dashed lines) have been selectively removed usingthe erase mask 509 of FIG. 2 b. However, the dummy regions 110 and 111are present on transistor region 100. The effective gate pitch oftransistor regions 100 is thus reduced resulting in transistors of lowerperformance locally. For example, such a structure may be needed iftransistor region 100 is neighboring a density sensitive region.

FIG. 13 illustrates a fourth embodiment of the semiconductor devicemanufactured in accordance with an embodiment of the present invention.In FIG. 13, the transistor regions 100 and 200 comprise transistorarrays 100 a and 200 a containing minimum pitch transistors 101-103 and201-203. However, the dummy gates 20 (regions 110 and 111) are presentonly on transistor region 100. The dummy regions 210 and 211 have beenremoved from transistor region 200 using the second erase mask. Althoughonly three transistor gates are drawn, the transistor arrays 100 a and200 a in transistor regions 100 and 200 may comprise any number oftransistors.

In embodiments described so far, the location of the erase mask step inthe process flow is only of minor importance. For example, in mostcases, the dummy regions may be removed either immediately after thefirst exposure as shown in FIG. 3, or in a subsequent process step afterthe gate stack formation. In preferred embodiments, the erase mask stepimmediately follows the first mask step to reduce errors for examplefrom misalignment. However, in some embodiments the exact location ofthe second mask step can be important. FIG. 14 illustrates such anexample in the formation of a transistor with source/drain stressors 43.The source/drain regions could be SiGe layers deposited to compressivelystrain the silicon channel of p-channel transistors.

The stressors 43 are deposited by etching a region of the siliconsubstrate 99 after gate stack formation. In the embodiment shown in FIG.14, the second mask step may be performed after formation of SiGestressors but before the formation of source/drain implants. Thetransistor regions 100 and 200 are shown after the completion of thesecond erase step. Region 100 comprises an isolated transistor 101 andregion 200 contain transistor arrays 201-204. However, as shown in FIG.14, both regions 100 and 200 have similar formation of stressor layers43. In other words, the relative ratio of stressor layers 43 andsubstrate 99 is similar. Hence, such an embodiment would generateuniform strain profiles (for example, minimize differences in channelstrain between isolated and dense transistor layouts), and would alsominimize any negative impact arising from non-uniform formation ofsource/drain regions.

An embodiment of the semiconductor device manufactured in accordancewith an embodiment of the present invention is described in FIG. 15,which includes FIGS. 15 a and 15 b. In various embodiments described inFIG. 15, the second mask 509 of FIG. 2 b, also referred to as an eraseor cut mask, selectively removes regions of the dummy lines “locally.”For example, to enhance strain, the first dummy line adjacent to a gateline is removed. However, to enable process yields, the dummy lines 20are retained further into the isolation region 26. FIG. 15 a illustratesan example using this approach for an isolated device 201 and FIG. 15 billustrates this approach for a densely packed transistor array.

In FIG. 15 a, the active poly line 10 forms transistor 201, whereas thedummy lines 20 form dummy features 210-213. The dummy features 214 and215 (shown by dashed lines) have been selectively removed to increasethe distance d₁ between the active poly line 10 or transistor gate 201and the next dummy feature 211 or 212. However, neighboring dummyfeatures may not be removed, for example, to maintain uniform etchingfrom a subsequent CMP process. For example, the distance d₂ betweendummy features 212 and 213 and distance d₃ between features 210 and 211may be smaller than distance d₁. Thus, this illustrates an embodimentwhere transistor performance metrics are co-optimized with processmargin and yield issues.

In FIG. 15 b, the active poly line 10 forms transistors 101-103separated by the pitch p, whereas the dummy lines 20 form dummy features210-213. The dummy features 214 and 215 (shown by dashed lines) havebeen selectively removed to increase the distance d₁ between the activepoly line 10 or transistor 101 and the next dummy feature 211 ortransistor 103 to next dummy feature 212. As in FIG. 13 a, the distanced₂ between dummy features 212 and 213 and the distance d₃ betweenfeatures 210 and 211 may be smaller than d₁.

Although in FIG. 15, only one dummy region per side is removed, inalternate embodiments, more neighboring dummy regions may also beremoved. For example, in FIG. 15 b, dummy regions 211 and 212 may alsobe removed in some embodiments.

In various embodiments discussed so far, the use of a selective doublepatterning technique illustrated creation of symmetric devices. However,as shown in various embodiments in FIG. 16, asymmetric devices may becreated using this technique. Asymmetric devices are useful in a numberof circuits such as, for example, in analog applications.

FIG. 16 a illustrates the formation of an asymmetric transistor due toselective removal of printing assist patterns from transistor regions.The transistor 201 has a neighboring dummy gate line 20 on one side (thedrain 57 side). The dummy gate line 211 on the source 55 side has beenremoved. Consequently, the strain profile on the source 55 side isdifferent from the strain profile in the drain 57 side of the transistor201. It is well known that transistor performance especially for scaleddevices depends on carrier injection velocity on the source side(location of this region within the channel region 18 is shown by thearrow), which in turn determines the source side mobility or strain inthe transistor. Consequently, the illustrated device is asymmetric,i.e., it behaves differently if biased from the drain side versus biasedfrom the source side. Further this asymmetry is present only during the“ON” phase. In other words, threshold of the asymmetric devices are notsignificantly asymmetric in the two scenarios.

In a different embodiment shown in FIG. 16 b, the transistors 101-103are formed with a pitch p (distance between them). The printing assistfeatures are formed irregularly, for example the distance d₁ of thetransistor 101 to the adjacent dummy feature 210 is different from thedistance d₂ of the transistor 103 to the adjacent transistor 211.Consequently, this forms an asymmetric stack of transistors.

Although embodiments of the present invention and their advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.For example, it will be readily understood by those skilled in the artthat many of the features, functions, processes, and materials describedherein may be varied while remaining within the scope of the presentinvention. Moreover, the scope of the present application is notintended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the present invention,processes, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed, thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present invention. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

1. A method of manufacturing a semiconductor device, the methodcomprising: depositing a conductive material over a semiconductorsubstrate, the conductive material covering a first and a second region;using a first mask to pattern the conductive material, thereby forming aplurality of first and second features on the first region and aplurality of first and second features on the second region, wherein thefirst features comprise active components of the semiconductor deviceand the second features comprise dummy features; and using a second maskto remove at least some of the second features from the first region andbut not from the second region.
 2. The method of claim 1, wherein thefirst region comprises an isolated transistor region and the secondregion comprises a densely packed transistor region.
 3. The method ofclaim 1, wherein the first region and the second region each comprise anisolated transistor region.
 4. The method of claim 1, wherein the firstregion and the second region each comprise a densely packed transistorregion.
 5. A method of manufacturing a semiconductor device, the methodcomprising: depositing a gate material over a semiconductor substrate;using a first mask to pattern the gate material thereby forming aplurality of first and second features over the semiconductor substrate,wherein the first features comprise gate electrodes of the semiconductordevice and the second features comprise dummy electrodes; and using asecond mask to remove selected ones of the plurality of second features,wherein the selected ones are based on a criterion that depends on alocation of each second feature relative to at least one of the firstfeatures.
 6. The method of claim 5, wherein each gate electrodecomprises a gate electrode of an isolated transistor.
 7. The method ofclaim 5, wherein using the second mask to remove selected ones of theplurality of second features comprises removing second featuresneighboring first features.
 8. The method of claim 5, wherein using thesecond mask to remove selected ones of the plurality of second featurescomprises removing first and second nearest neighbors to the firstfeatures.
 9. The method of claim 5, wherein using the second mask toremove selected ones of the plurality of second features creates anasymmetric semiconductor device.
 10. The method of claim 5, wherein thecriterion for removing the selected ones of the plurality of secondfeatures comprises improving electrical performance of the semiconductordevice.
 11. The method of claim 5, wherein the criterion for removingthe selected ones of the plurality of second features comprisesimproving electrical performance of the semiconductor device withoutincreasing layout area.
 12. The method of claim 5, wherein the criterionfor removing the selected ones of the plurality of second featurescomprises improving electrical performance of the semiconductor devicewithout decreasing process yield.
 13. A method of manufacturing asemiconductor device, the method comprising: forming isolation regionsadjacent to active regions on a semiconductor substrate; depositing aconductive material over the semiconductor substrate; using a first maskto pattern the conductive material, thereby forming a plurality offeatures on the semiconductor substrate, wherein the features compriseconductive lines over the active regions and the isolation regions; andusing a second mask to remove some but not all of the conductive linesfrom over the isolation regions.
 14. The method of claim 13, wherein theconductive lines over the active regions comprise gate electrodes ofisolated transistors.
 15. The method of claim 13, wherein using thesecond mask comprises removing the features neighboring the activeregions.
 16. The method of claim 13, wherein using the second maskcomprises removing first and second nearest neighbor features from theactive regions.
 17. The method of claim 13, wherein using the secondmask comprises forming an asymmetric semiconductor device.